Iridescences: The Physical Colors of Insects

Iridescences: The Physical Colors Of Insectswas written by Serge Berthier, a French physicist and materials scientist with a passion for beautiful insects. Iridescence, or more specifically, structural colors, are produced by refraction from microstructures smaller than the wavelength of light, such as the rainbow effect produced by compact discs. The gorgeous blue in Morpho butterflies and the bright metallic gold of many tropical beetles are the result of submicron chitinous structures in the cuticle. These structural, refractive mechanisms of generating color are fundamentally different from the more familiar and intuitive mechanism of coloration via pigment, or the differential absorption of wavelengths of light by pigment molecules. In addition to being an important aspect of insect natural history, structural colors are of interest to physicists and materials scientists because many of the tiny structures responsible for some of the more impressive insect phenomena are still impossible to fabricate. In some cases, the study of insect structural colors has revealed optical phenomena that were never imagined or characterized by physicists. Berthier explores and explains color-generating structures and mechanisms in insects, as well as other relevant topics in natural history and biological optics.

The most useful and unique portion of the book is the three chapters detailing the impressive array of structures that generate color in beetles and butterflies. Although this information is scattered throughout the literature, to my knowledge, this is the first time it has been assembled in one place using clear, specific biological examples. There is one chapter each on one-, two-, and three-dimensional structural colors that explains the very different physical mechanisms by which each dimension of geometry produces color, with photographs and examples of each type in insects.

In addition to this core information, the book contains chapters on color space and color theory, the anatomy of butterfly wings and beetle cuticle, the natural history of butterfly coloration, insect thermoregulation via pigments and structural colors, and a primer on biological pigments. The rest of the material in the book is helpful, but it can be found elsewhere, and essentially supports and provides background for the chapters on the dimensional aspects of structural color.

The major flaw of the book is the generally sloppy editing. First, strange French–English hybrid grammar appears in almost every sentence. Most of the time, the important ideas are communicated, and the French flavor occasionally adds a little fun to the narrative, but it was often a struggle to read through the awkward phrasings and then parse what the author meant. Occasionally, the careless translation obscures real physical meaning, such as the author’s repeated use of the term “dispersion” where he apparently means “diffusion”—these terms have physical definitions in English that are evidently different from the French cognates. In another example, the term “photonic microscope” appears throughout the book, which at first I thought might be a sophisticated piece of physics equipment I’d never heard of, but evidently means “light microscope.” In addition, the editing of the figures is problematic—in several captions, panels are labeled “top” and “center” when they are sitting left and right of one another, and vice versa. Other figures are miscaptioned.

The book strangely has no reference section, although the scientific work of other authors is implicitly discussed. For instance, Berthier describes a concentration gradient model of butterfly eyespot development that is evidently from the work of Fred Nijhout and others, but those authors are never mentioned in the main text or any bibliography. This is problematic not only because these ideas should be properly acknowledged, but also because it is difficult for readers to know where to turn if they would like more information about a topic or to verify the author’s claims.

Underneath the sometimes awkward translation and editing problems, however, is a very good introduction and reference for biologists who are interested in the optical aspects of their study organisms, as well as a primer in animal diversity and biological mechanisms for physical scientists. On a second reading, after deciphering the translation and captioning issues, I realized what a valuable collection of physics, biology, and unpublished micrographs this work could be as a resource for scientists interested in the interface of photonics and biology, or as a text for a seminar course on the subject.

The many photographs of butterfly scales in transmission illumination, immersed in index-matching fluid, and in SEM are an invaluable collection that would take many hours or days to assemble from other sources, and in many cases may not be available in the literature. These photographs are generally of very high quality, and are beautifully reproduced on the book’s glossy paper. In most cases, the photographs are more informative than the text in understanding the topics at hand. Especially effective and useful are the nested photographs of the same cuticular structure shown at several different size scales. Having these marvelous photographs collected in a single place is probably worth the book’s price for scientists interested in this topic.

I found most of the physical and mathematical explanations in the text to be accessible to a physics-minded biologist. For most of the optical subjects discussed, there is a “back of the envelope” intuitive diagram of a given phenomenon, as well as equations to describe the phenomenon, although the author avoids long mathematical derivations. This approach provides some useful physical insight to biologists who have likely forgotten any background they may have had in matrix math, but it also gives context and an analytical starting point to the physicists in the audience.

This book could work well as the foundation of an interdisciplinary graduate seminar or advanced undergraduate course on structural color and photonics, as long as the editing and referencing issues were considered beforehand. It would also be useful to people working at the interface of physics and biology, as a reference and source of photographic examples of many different biological optical phenomena, or as an introduction to these topics for someone with traditional biology training who hopes to learn some optics, and vice versa.